The application of inkjet printers varies widely with the movement in recent years toward those having higher functions. Their application is expanding further, for example, not only as those for personal use, office use, business use, recording, colored presentation and color photos but also as industrial inkjet printers. For industrial inkjet printers in particular, it is required especially to meet high-speed printing. Concerning water-based pigment inks for use in such inkjet printers, the ultrafine division of pigment particles has proceeded to make improvements in brilliance, color clearness, color density and so on. On the other hand, the miniaturization of ejected droplets (ink droplets) has also proceeded to meet the movement toward printing of higher speed and higher quality through improvements in printer apparatus. These improvements have brought about excellent high-quality image quality for converted papers for inkjet printing, especially photographic papers, wide format papers, and the like. Nonetheless, these improvements have not advanced to such an extent as to apply the inkjet printing method to printing on papers of various qualities. A new problem has arisen especially in that depending on the kind of paper, the printed ink may flake off when rubbed.
In recent years, printing is often applied industrially to various film-shaped base materials made of polyvinyl chloride, polyolefins, PET, polystyrene and the like. It is, therefore, necessary to form printed ink films (printed ink) of high adhesiveness on these film-shaped base materials.
To solve the above-described various problems, a binder that forms printed ink films is added in an ink. There is, accordingly, a need for a film-forming binder that can meet papers of various qualities, can form printed ink films with high adhesiveness to various films, and has good inkjet printability (high-speed printability, ejection stability). It is to be noted that papers and films may hereinafter be collectively referred also to as “base materials”.
Incidentally, inks may dry up in printer heads. Inkjet inks are, therefore, required to have resolubility such that they can be easily removed by dissolving or dispersing them with a cleaning solution or the like even when they dry up and stick in the heads.
Under such background circumstances, a variety of film-forming binders have been developed in recent years.
Described specifically, solutions or emulsions that contain water-soluble, acrylic, urethane, vinyl or like polymers, which can act as ordinary film-forming components, are known as binders. However, such water-soluble polymers are soluble in aqueous media for water-based pigment inks, and therefore involve a problem that the inks become higher in viscosity. Further, such water-soluble polymers include those provided with improved water-solubility by containing carboxyl groups at a high concentration and neutralizing them with a neutralizing alkaline material. These polymers are certainly provided with good resolubility, but are mostly dissolved in aqueous media. As a consequence, the inks tend to become still higher in viscosity. Furthermore, the inks may show non-Newtonian viscosity, and may not be ejected smoothly from heads. In addition, because of the inclusion of many carboxyl groups in the polymers, the resulting films may not be provided with good water resistance.
Attempts have also been made to lower the concentration of carboxyl groups in contrast to the above-described approach such that the polymer is lowered in solubility and is particulated to provide a water dispersion or emulsion (polymer emulsion) for use as a binder. Such a binder can provide an ink with lower viscosity, and can also provide the resulting film with good water resistance owing to the low concentration of carboxyl groups in the polymer. Moreover, the polymer itself is hydrophilic so that the resulting film is provided with improved adhesiveness to a base material. However, such a polymer is poor in water solubility, and therefore tends to provide the resulting ink with low resolubility. It is, therefore, necessary to use a water-soluble solvent.
Also proposed as polymer emulsions useful as binders include an emulsion which can be obtained by emulsion polymerization making use of a conventionally-known, low molecular surfactant or reactive surfactant; and self-emulsifiable emulsions each of which can be obtained by conducting polymerization with a carboxyl-containing monomer added in a small amount and then conducting neutralization (see Patent Documents 1 to 3). As the polymers contained in these emulsions are high in molecular weight, the resulting films are provided with improved adhesiveness to base materials. However, the polymers contained in these emulsions become hardly soluble in aqueous media when they are dried up into films. The inks, therefore, tend to be provided with low resolubility.